The invention is directed to a process for the purification of industrial gases or industrial waste gases which contain hydrogen cyanide, as well as frequently ammonia, and in which there can also be present carbon oxides, hydrocarbons of various structures, hydrogen sulfide, sulfur oxides as well as metal containing dusts.
These types of gases are obtained, e.g., in blast-furnace processes, acrylonitrile syntheses, garbage pyrolyses or other chemical processes and are purified by washing with aqueous formaldehyde.
The invention is directed especially to controlled dosing of the aqueous formaldehyde electrometrically.
The purification of industrial gases or waste gases is required for various reasons. On the one hand, it serves to separate dusts from valuable gases, as occurs in blast-furnace processes or garbage pyrolyses, on the other hand waste gases, as, e.g., are formed in the synthesis of acrylonitrile or other chemical syntheses, must be freed of toxic materials before passing into the atmosphere. The removal of dust from blast furnace gas frequently was carried out in a continuous-flow process. Thereby, the wash water was led after utilization over a circular thickener which simultaneously served as detoxification vessel and the clear decantate led to the sewer system or receiving stream of the sewer.
Since very large amounts of water are needed for this purpose, at present the recirculating process is preferred. Thereby, the wash water is treated with a flocculating agent after the gas washing, led over a circular thickener and the clear decantate supplied again to the washing circuit via an evaporation cooler. Fresh water is added only to compensate for evaporation losses respectively to lower the hardness of the water. Consequently, only a fraction of the wash water is used for detoxification; however, this has a substantially higher noxious material content than the wash water from the continuous-flow process. Only in very rare cases, there is carried out a pH regulation of the wash water in the circuit, so that the pH is in the neutral range. As a result, a large part of the cyanide is carried out of the evaporation cooler in the form of hydrogen cyanide.
In German OS No. 2460927 there is described a two-step process for treatment of blast furnace gas wash water which depends on measuring the content of cyanide before the sedimentation plant and then adding 20 to 70% of the amount of formaldehyde required to react stoichiometrically with cyanide forming glycol o-nitrile. The pH of the entire blast-furnace gas wash water thereby must be adjusted to between 8 and 10.
A very good intermixing in the area of addition of formaldehyde is essential for the process in order to avoid a local excess of formaldehyde which would lead to reaction with other materials, such as ammonium ions. Excesses of formaldehyde, including local excesses, however, should be avoided in every case. Therein is seen a distinctiveness of the process.
Then in a second step there is added the residual portion of the stoichiometric amount of formaldehyde needed to react with hydrogen cyanide, after the gravity separator and removal of the polymers of glycolonitrile formed in the first step, in order to detoxify complex heavy metal cyanides. For this type of process, however, there would be needed exact information to control the addition of formaldehyde, in order to avoid, e.g., the stated impermissible local excesses of formaldehyde in the presence of reactive ions, such as, e.g., ammonium ions. This type of information, however, is completely missing. However, it is necessary in regard to the strongly fluctuating cyanide content in the wash water of blast-furnace gases, e.g., according to the process for production of iron between 0.1 and 202 mg CN.sup.- /l of wash water, since a dosing according to empirical values then can no longer be carried out. Entirely apart from this even during the production itself there can occur significant fluctuations in the cyanide content. A process for the detoxification of waste water containing high concentrations of cyanide with an alkaline formaldehyde solution whose pH is at least 8 and which preferably is employed in excess over the stoichiometrically necessary amount is described in German OS No. 2119119. The carrying out of the process is technically expensive, both because of the multi-hour heating of the waste water to the boiling temperature and the standing for days at room temperature. In none of the cases is there produced a sufficient detoxification. Thus, the residual cyanide content after boiling the waste water for 2 hours is 0.5 mg CN.sup.- /l and after standing for 50 hours at room temperature is 8 mg CN.sup.- /l.
A process primarily for the detoxification of waste waters of acrylonitrile plants which then are subsequently supplied to a biological sewage treatment plant consists of using a formaldehyde solution having a pH of 3 or lower (German Pat. No. 2202660), namely in molar excess; preferably 1.5 to 4 moles of formaldehyde are employed per mole of cyanide.
This continuous process, however, can only be controlled by a wet analytical determination of the cyanide content in the waste water, since according to the data in the patent the establishment of the equilibrium takes place so quickly that it cannot be measured with a silver iodide electrode.
However, it cannot be seen from the examples how the strongly fluctuating amounts of waste water--there are mentioned amounts between 20 and 40 m.sup.3 /h having cyanide contents of 20 to 300 ppm--can be detoxified without problems without electrometric control, in any event it can be seen from the examples that even upon addition of four times the amount of formaldehyde, based on the cyanide content, the limit on cyanide ions of less than 0.1 mg CN.sup.- /l required today in no case is reached. Glycolonitrile according to this process should not be formed, but there is an unknown reaction, apparently with formation of pyrimidones.